Prediction of magnetic ordering of FePd by He+ irradiation

(click to enlarge)

Ultra-high storage capacities (> 1 Tbit.in-2) of magnetic harddisks require the transition to perpendicular recording with nanomagnets consisting of highly anisotropic magnetic materials like FePt or FePd. However, only the L10-superstructure shows this magnetic property. In the Fe-Pt phase diagram it is indicated that synthesis of FePt (e.g. by layer deposition) leads usually to crystalline fcc lattices with chemical disorder, i.e. to the A1 structure. The A1 structure has a high equilibrium temperature of more than 1000°C. The difference of enthalpy for the transition to the L10 superstructure is small. This is one reason, why extremely long heat treatments can be necessary for the L10 ordering process. Our experimental partners have discovered that time and temperature of heat treatment for L10 ordering can be reduced strongly by simultaneous irradiation of the FePd layer with He+ ions.

At the FZD it has been found by means of atomistic simulations that the ordering process is accelerated by He+-irradiation-induced vacancy formation. The experimental layer stack is shown above. The essential FePd layer is sandwiched between Pd layers. Without irradiation the L10 ordering is hampered by the high formation energy of vacancies, which control the kinetics. Under irradiation vacancies are mainly formed by collisions of He+ ions with metal atoms which lowers the ordering temperature significantly, in good agreement with experiments. The Monte-Carlo simulation shown here were carried out with experimentally determined atomic energies. After 0.04 displacements per atom (dpa) one can see an ordering of Pd atoms (black sqares) into monoatomic (100) layers separated by Fe layers. There exist different variants of the L10 structure with perpendicular and horizontal lines as well as checker board pattern (see Fig. for 0.04 dpa).

(click to enlarge)

(click to enlarge)

For novel harddisks (»perpendicular recording«) the strong magnetic axis has to be normal to the layer, i.e. only one variant of the L10 superstructure is allowed. This was achieved experimentally by ion irradiation of FePd layers. Computer simulations based on experimental data led us to a deeper understanding of the selection mechanism of the variant: During the deposition of the FePd layer a small disturbance of the isotropy of the chemical disorder forms (the directional short range order DSRO has been measured) which causes during the L10 ordering a selection of the axes of strong anisotropy normal to the surface (see the lower right figure).

The He+ irradiation is less efficient for FePt nanoparticles because the vacancies escape faster to the surface (see in the lower figure a sequence of snapshot of the formation of the L10 superstructure; arbitrary Monte-Carlo time unit). Mostly single-variant-particles form after long lasting irradiation due to symmetry breaking by fluctuations. It should be noted that Pd covers the particle surface.